Geology: that sinking feeling

England’s record rainfall last winter triggered the appearance of sinkholes in unprecedented numbers. “Terrifying holes … are opening up all over Britain,” reported the Daily Mail. Or “Brits suffer sinkhole hell”, as The Sun put it.

Over three weeks in late February and early March sudden subsidence caused more than 25 large holes to open up across southern Britain. The British Geological Survey, which was inundated by enquiries, usually hears about no more than half a dozen sinkholes during a year. Now that normality has returned, two BGS scientists, Andy Farrant and Tony Cooper, have published an analysis of the sinkhole hazard across the UK in Geoscientist.

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Heavy rainfall or surface flooding can trigger subsidence in two ways. The additional weight of saturated ground may increase the loading on the surface material capping a cavity, which then collapses; and more water flowing through a cavity can flush out the looser sediments filling it, creating a void into which the cap collapses.

Both natural and man-made cavities can cause sinkholes. Most of those that appeared recently in southern England and the Midlands are due to the collapse of old shafts or mines. Last winter’s largest new UK sinkhole, 49m wide and 40m deep, opened up in fields near Foolow, Derbyshire, when old lead workings caved in. According to Farrant: “There are any number of old artificial cavities in the UK, far more than people realise.”

Ancient chalk workings often cause trouble. From medieval times until the 19th century, chalk was dug intensively in parts of southeast England – to lime soil, to obtain flint and as an ingredient in brick making. “Nowadays we would probably transport the chalk from surface outcrops, which might be a long way away, but then it was often easier to sink a shaft through the sandy or clay soil covering the chalk and haul it up,” Farrant says.

The resulting cavity, known as a denehole, typically had several excavated chambers that were reached down an excavated shaft 20m deep. Later the top of the shaft was capped or blocked and forgotten. A sinkhole that appeared last winter in the grounds of Rainham Mark Grammar School in Gillingham, Kent, had ancient bricks at the bottom – suggesting that it was a chalk denehole topped with a brick arch that had collapsed.

Natural sinkholes occur more frequently in northern England. The most vulnerable area is around Ripon, North Yorkshire, where the underlying rock is gypsum (calcium sulphate). Gypsum dissolves faster than the more common limestone (calcium carbonate) as water runs through it, leaving cavities whose roofs eventually fall in.

According to the BGS: “At least 30 major collapses have occurred in the last 150 years [around Ripon] some of which have affected property and infrastructure.”

Geological conditions cause more spectacular – and dangerous – sinkholes to appear in overseas locations than in the UK. Florida is particularly vulnerable. Some central and western parts of the state have a limestone “karst” landscape riddled with underground caverns capped with thin roofs liable to collapse; extensive urban and suburban development has exacerbated the risk by changing the natural pattern of subterranean water flows. Last year a resident of Tampa, Jeff Bush, died when a sinkhole swallowed up his home.

Most of the holes that appeared in England are due to the collapse of old shafts or mines

The deepest urban sinkhole in recent years opened in Guatemala City in May 2010 following a tropical storm. This created a cylindrical hole 20m across and 100m deep. Here the cause was water flowing underground through soft and unconsolidated volcanic deposits. Nowhere in Britain has loose volcanic strata like those in Guatemala.

The BGS is building a database and maps of sinkhole risks across the UK, and Farrant advises people building houses or infrastructure in vulnerable areas to carry out their own local research. “Good site investigation should be able to mitigate the risk,” he says.

Photographs: Reuters; Getty

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